1. Field of the Invention
The present invention relates to an ink-jet printhead. More particularly, the present invention relates to a thermally-driven ink-jet printhead having an improved structure that is capable of preventing cavitation damage to a heater.
2. Description of the Related Art
In general, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of ink at a desired position on a recording sheet. Ink-jet printheads are generally categorized into two types depending on which ink ejection mechanism is used. A first type is a thermally-driven ink-jet printhead in which a source of heat is employed to form and expand bubbles in ink to cause an ink droplet to be ejected due to the expansive force of the formed bubble. A second type is a piezoelectrically-driven ink-jet printhead, in which an ink droplet is ejected by a pressure applied to the ink and a change in ink volume due to a deformation of a piezoelectric element.
An ink droplet ejection mechanism of a thermal ink-jet printhead will now be explained in detail. When a pulse current is applied to a heater, which includes a heating resistor, the heater generates heat and ink near the heater is instantaneously heated to approximately 300° C., thereby boiling the ink. The boiling of the ink causes bubbles to be generated, and exert pressure on ink filling an ink chamber. As a result, ink around a nozzle is ejected from the ink chamber in the form of a droplet through the nozzle.
A thermal ink-jet printhead is classified into a top-shooting type, a side-shooting type, and a back-shooting type depending on a bubble growing direction and a droplet ejection direction. In a top-shooting type of printhead, bubbles grow in the same direction in which ink droplets are ejected. In a side-shooting type of printhead, bubbles grow in a direction perpendicular to a direction in which ink droplets are ejected. In a back-shooting type of printhead, bubbles grow in a direction opposite to a direction in which ink droplets are ejected.
An ink-jet printhead using the thermal driving method should satisfy the following requirements. First, manufacturing of the ink-jet printheads should be simple, costs should be low, and should facilitate mass production thereof. Second, in order to obtain a high-quality image, cross talk between adjacent nozzles should be suppressed while a distance between adjacent nozzles should be narrow; that is, in order to increase dots per inch (DPI), a plurality of nozzles should be densely positioned. Third, in order to perform a high-speed printing operation, a period in which the ink chamber is refilled with ink after ink has been ejected from the ink chamber should be as short as possible and the cooling of heated ink and heater should be performed quickly to increase a driving frequency.
FIG. 1 illustrates a partial cut-away perspective view of a conventional thermally-driven ink-jet printhead. FIG. 2 illustrates a cross-sectional view of the conventional thermally-driven ink-jet printhead shown in FIG. 1.
The ink-jet printhead shown in FIG. 1 includes a base plate 10 formed of a plurality of material layers stacked on a substrate, a passage plate 20, which is stacked on the base plate 10 and forms an ink chamber 22 and an ink passage 24, and a nozzle plate 30 stacked on the passage plate 20. Ink is filled in the ink chamber 22, and a heater (13 of FIG. 2) for generating bubbles by heating ink is disposed below the ink chamber 22. The ink passage 24 is a path through which ink is supplied to the ink chamber 22 and which provides flow communication from an ink reservoir (not shown). A plurality of nozzles 32, through which ink is ejected, is formed at a position of the nozzle plate 30 corresponding to each ink chamber 22.
The vertical structure of the conventional inkjet printhead having the above structure will now be described with reference to FIG. 2.
Referring to FIG. 2, an insulating layer 12 is formed on a substrate 11 formed of silicon, to provide insulation between a heater 13 and the substrate 11. The insulating layer 12 is formed by depositing a silicon oxide layer on the substrate 11. The heater 13 for generating a bubble 42 by heating ink 41 in an ink chamber 22 is formed on the insulating layer 12. The heater 13 is formed by depositing tantalum nitride (TaN) or a tantalum-aluminum (TaAl) alloy on the insulating layer 12 in a thin film shape. A conductor 14 for applying current to the heater 13 is formed on the heater 13. The conductor 14 is made of aluminum or aluminum alloy.
A passivation layer 15 for protecting the heater 13 and the conductor 14 is formed on the heater 13 and the conductor 14. The passivation layer 15 prevents the heater 13 and the conductor 14 from oxidizing or directly contacting the ink 41, and is formed by depositing silicon nitride. In addition, an anti-cavitation layer 16, on which the ink chamber 22 is to be formed, is formed on the passivation layer 15. The anti-cavitation layer 16 is formed of metal, e.g., tantalum (Ta).
A passage plate 20 for forming the ink chamber 22 and the ink passage 24 is stacked on a base plate 10 formed of a plurality of material layers stacked on the substrate 11. A nozzle plate 30 having a nozzle 32 is stacked on the passage plate 20.
In the above structure, if a pulse current is supplied to the heater 13 and heat is generated by the heater 13, the ink 41 filling the ink chamber 22 boils, and a bubble 42 is generated. The bubble 42 expands continuously and applies pressure to the ink 41 in the ink chamber 22. As a result, an ink droplet 41′ is ejected through the nozzle 32.
In the above-described conventional thermally-driven ink-jet printhead, however, a supply of energy from the heater 13 is interrupted, and heat is dissipated to the ink 41 around the bubble 42. As a result, the expanding bubble 42 contracts rapidly. When the bubble 42 contracts and collapses in this manner, a very high pressure is applied to a portion of the ink chamber 22 where the bubble 42 finally collapses. As a result, the heater 13 and the passivation layer 15 covering the heater 13 in the vicinity of the collapse are damaged. This damage is referred to as cavitation damage, and points where the bubble 42 collapses, i.e., points where the cavitation damage occurs, are referred to as cavitation points. Cavitation damage occurs repeatedly during every ejection cycle and becomes severe. As a result, the formation of the bubble 42 varies, the reliability of normal operation of a printhead decreases, and the lifespan of the printhead is shortened.
Conventionally, in order to protect the heater 13 and the passivation layer 15 from cavitation damage, a thick anticavitation layer 16 is stacked above the heater 13. However, in this case, more energy is required to heat the ink 41 in the ink chamber 22. As a result, the printhead is overheated, which adversely affects a driving frequency of the printhead.
A variety of heater structures have been recently proposed to prevent problems related to cavitation damage. Two examples of such heater structures are shown in FIGS. 3 and 4. FIG. 3 illustrates a plan view of an example of a conventional heater structure for preventing cavitation damage. FIG. 4 illustrates a plan view of another example of a conventional heater structure for preventing cavitation damage.
Referring to FIG. 3, a conductor 57 is connected to opposite sides of a heater 50 formed on a silicon substrate 55. A conductive area 53, formed of a metallic conductive material, is formed at a center of the heater 50. Resultantly, a bubble is not generated at a central area of the heater 50, but rather a ring-shaped bubble is formed at a peripheral area of the heater 50. The ring-shaped bubble contracts and collapses in such a way that a cavitation shock is dispersed to a surface of the heater 50. However, even though the cavitation shock is dispersed to the surface of the heater 50, if the cavitation shock is repeatedly applied to the surface of the heater 50, damage to the heater 50 cannot be avoided. In addition, in order to eject an ink droplet having a predetermined amount of ink, i.e., a predetermined volume, a bubble corresponding to the predetermined amount of ink is required. Since the bubble is not generated at the central area of the heater 50, the entire size of the heater 50 is required to increase. As a result, a size of an ink chamber increases, which results in poor fluid, i.e., ink, movement, thereby making it difficult to increase a driving frequency.
In FIG. 4, conductors 65 and 66 are connected to both sides of a heater 62. A hollow portion 70 is formed at a center of the heater 62. Accordingly, the heater 62 has a ring shape to surround the hollow portion 70, and a bubble is not generated in the hollow portion 70. However, current does not uniformly flow through the ring-shaped heater 62. Thus, an amount of heat generation is not constant. In addition, since the entire size of the heater 62 significantly increases to permit the formation of the hollow portion 70, it is again difficult to increase a driving frequency, as in the heater shown in FIG. 3.